1. Field of the Invention
This invention relates to the production of petroleum hydrocarbons from a subsurface reservoir which overlies a water column.
2. Description of the Prior Art
Often a petroleum hydrocarbon reservoir will overlay a substantially water saturated zone. When wells are drilled through this type of formation and production of hydrocarbons is begun, a problem referred to as water coning is often encountered. Water coning is a term given to the mechanism underlying the entry of bottom waters into producing wells. Petroleum hydrocarbons are often produced from porous subsurface formations which overlay a substantially water saturated porous formation. Under static conditions the water, being of greater density than the hydrocarbons, remains beneath and at the bottom of the hydrocarbon producing formation. However, at high rates of production of petroleum the upper boundry or surface of the substantially water saturated formation rises due to the increased flow of petroleum hydrocarbons into the wellbore. The wellbore usually extends into the liquid petroleum producing formation immediately adjacent and above the substantially water saturated formation. The rise of water into the hydrocarbon producing formation and into the wellbore represents a dynamic effect in which the upward directed pressure gradients associated with the flow of hydrocarbons into the producing wellbore are able to balance the hydrostatic head of the resulting elevated water column.
Various methods have been suggested to eliminate or reduce the water coning phenomenon. These methods have included reducing the well penetration into the hydrocarbon producing formation so that higher production rates are possible without at the same time experiencing a relatively increased production of water. Another method which has been suggested is to bottom the producing well into a substantially water impermeable formation. These indicated methods, however, cannot be successfully employed in all instances to eliminate or reduce water coning. Certain underground hydrocarbon producing formations are only relatively few feet in thickness. Accordingly, reducing the wellbore penetration to such a formation would unduly restrict the recovery of petroleum hydrocarbons therefrom. Other hydrocarbon producing formations do not have associated therewith an immediately underlying water impermeable formation such as a shale stringer.
One prior art method which has been shown to have some success in reducing the water coning problem is called "single wellbore cycling." In single wellbore cycling described in U.S. Pat. No. 2,832,416 the producing well is shut in for a period of time necessary to approach equilibrium conditions in the subsurface producing formations. During this period of time the water cone tends to flatten out. The time required depends on various circumstances and may be three hours to a month. After essentially equilibrium conditions have been reached, a liquid petroleum fraction is injected into and cycled within the hydrocarbon producing formation. After a sufficient amount of liquid petroleum has been injected and cycled to force the water in the petroleum producing formation to a substantial radial distance outwardly from the wellbore the injection of liquid petroleum is continued and petroleum production is commenced. This technique drastically changes the normal pressure gradients set up around the wellbore and prevents premature invasion or coning of water into the petroleum producing formation. The simultaneous injection/producing is accomplished by completing the well in such a way that injection of fluid into the lower part of the formation near the water column may be accomplished without communicating inside the wellbore with the hydrocarbons and other fluids being produced through the same well from the upper part of the formation. This may be accomplished by several arrangements of tubing and packers for example. Obviously, the injection rate of the recycled petroleum liquid will have to be less than the petroleum production rate as measured at the well head so that there is a net production of petroleum from the formation.
While this single wellbore cycling method finds application in many instances, it is not altogether successful in certain reservoirs. The method of this invention is a vast improvement over the single wellbore cycling method described above.